a) Field of the Invention
The present invention relates to an optical system for non-flexible endoscopes.
b) Description of the Prior Art
A non-flexible endoscopes has a composition shown in FIG. 1, for example, or consists of an elongated insert section 11 which is to be inserted into a cavity of a living body or the like, and a grip section 12 which is to be located outside the cavity in use of the endoscope and is held by an operator's hand or an endoscope holder. Furthermore, an illumination system 13 which illuminates an object, and an observation optical system 14 which is used for observing and picking up an image of the object are disposed within a range from the insert section to the grip section.
A section of the observation optical system 14 which is disposed in the insert section consists of an objective optical system 1 which forms a real image of the object and a relay optical system 2 which transfers the image of the object formed by the objective optical system 1 into the grip section 12. This relay optical system transfers the image ordinarily three cycles. Furthermore, a section of the observation optical system 14 which is disposed in the grip section 12 is an eyepiece optical system which makes the transferred object of the image visible by an eye.
When the non-flexible endoscope is used for a surgical operation under endoscopic observation, it is necessary to observe an image of an object on a screen of a TV monitor and a TV camera for the non-flexible endoscope is attached to an eyepiece mount of the grip section. A TV camera is indispensable in a surgical field for digestive system. Therefore, a TV camera 17 for the non-flexible endoscope which comprises an image pickup optical system 15 and a solid-state image pickup device 16 is attached as shown in FIG. 1. In FIG. 1, a reference numeral 18 represents a camera control unit and a reference numeral 19 designates a TV monitor.
The conventional non-flexible endoscope described above is configured to maintain brightness of the optical system by transmitting an image while relaying it a plurality of cycles on the order of three times.
A non-flexible endoscope which is used for surgical operations under endoscopic observation has an insert section which ordinarily has an outside diameter not larger than 10 mm and a length on the order of 300 mm or longer. In order to transfer an image while maintaining required brightness through an optical system of such an insert section, it is necessary to enlarge an NA of a relay optical system by increasing a number of relay cycles. When required brightness is maintained by such means, however, the relay optical system produces aberrations in large mounts, thereby degrading an image quality. When the number of relay cycles is increased, the relay optical system uses a larger number of lens components, requires a higher manufacturing cost, and degrades an image quality due to manufacturing errors.
As an optical system which is configured to transfer an image a single relay cycle in order to correct these defects, there is known an optical system disclosed by Japanese Patent Kokai Publication No. Hei 7-325249. This optical system for non-flexible endoscopes cannot be configured as an optical system which has a wide field and is compatible with a non-flexible endoscope for oblique observation.
Even when a relay optical system is configured to transfer an image while relaying it a plurality of cycles, it is general that aberrations are not aggravated so much on an image transferred through a relay optical system as a whole so far as aberrations are corrected sufficiently favorably in a relay optical system for a single relay cycle. In order to correct aberrations favorably in the optical system for single relay cycle, however, it is necessary to compose the relay optical system of a larger number of lens components. When the relay lens system is composed of a larger number of lens components, the optical system comprises a larger number of lens surfaces, and a light bundle is reflected and scattered more times by the larger number of lens surfaces, thereby lowering brightness and contrast of an image. Furthermore, the larger number of lens components enhance a manufacturing cost of the relay lens system and increase undesirable factors such as manufacturing errors at a manufacturing stage which degrades an image quality. For this reason, it is necessary that a single cycle type relay optical system must have a composition as simple as possible so that it comprises a reduced number of lens components. When the single cycle type relay optical system is composed of a reduced number of lens components, however, the optical system is incapable of correcting aberrations favorably. Since a relay optical system which transfers an image a plurality of time has factors which are conflicting with one another, it is difficult to configure an optical system which has a simple composition, can be manufactured at a low cost and corrects aberrations favorably.
When an image is relayed a large number of cycles, aberrations are multiplied by the number of relay cycles in a relay optical system as a whole. When an image is to be transferred by a relay optical system for a definite distance, a larger number of relay cycles shorten a length of an optical system for a single relay cycle and make it inevitable to strengthen a power of each surface of the relay optical system, whereby aberrations are produced in larger amounts per relay cycle. When a number of relay cycles in a relay optical system is multiplied by an integer, aberrations are produced in the relay optical system in amounts larger than those multiplied by the integer.
An optical system for non-flexible endoscopes is configured to correct aberrations produced in a relay optical system with an objective optical system and so on which are other than a relay optical system so that aberrations are corrected favorably in the optical system for non-flexible endoscopes as a whole. However, it is preferable to reduce aberrations as far as possible in a relay optical system since aberrations produced by the relay optical system cannot be corrected sufficiently favorably under conditions such as a restricted number of lens components and a restricted outside diameter.
When a relay optical system is configured for an increased number of relay cycles, it uses an increased number of lens components and an increased number of lens surfaces, whereby the relay optical system lowers brightness and contrast, enhances manufacturing cost of the optical system and degrades an image quality. In order to correct these defects, it is desirable to configure a relay optical system for a single relay cycle.
A technical theme for a relay optical system which transfers an image a single relay cycle lies in maintenance of brightness. When a number of relay cycles is reduced in a relay optical system which transfers an image for a definite distance, its numerical aperture is reduced and its brightness is lowered.
As a conventional example which solves this problem, there is known an optical system disclosed by Japanese Patent Publication No. Hei 7-325249. This conventional example is configured for a single relay cycle but maintains brightness sufficient for practical use by adopting a long objective optical system, a short relay optical system and an observation optical system having a large NA.
Since this conventional example uses the long objective optical system, however, it has a defect that it cannot have a wide field angle and is incompatible with non-flexible endoscopes for oblique observation.
An optical system for non-flexible endoscopes uses an objective optical system which is mostly of the retrofocus type since the objective optical system must have a small outside diameter and a wide field angle. When a total length of the retrofocus type objective optical system is prolonged with its outside diameter kept unchanged, rays coming from marginal portions of a visual field have small angle relative to an optical axis at a location of a pupil, thereby making it necessary to shorten a focal length of a front diversing lens component. When the focal length of the front diverging lens component is shortened too much, however, a remarkably complicated lens composition must be adopted to correct aberrations produced in large amounts by the front diverging lens component with other sections of an observation optical system. Since an observation optical system which is practically usable cannot have a complicated composition and is incapable of favorably correcting aberrations. Furthermore, when a focal length of a front diverging lens component is restricted due to correction of aberrations, it is impossible to configure an optical system which has a large field angle. The conventional example of optical systems for non-flexible endoscopes cannot have a field angle larger than 70.degree. and it is preferable, when sufficiently favorable correction of aberrations is taken into consideration, that an optical system has a field angle not exceeding 60.degree..
Since a field angle larger than 70.degree. is mostly desired for non-flexible endoscopes to be used for surgical operations under endoscopic observation, the conventional example of optical system described above cannot be used for these non-flexible endoscopes.
This conventional example can hardly have the required field angle as described above.
The conventional example of optical system can hardly have the required field angle as described above and has a pupil of the objective optical system which is located in the vicinity of a center of the objective lens system so that rays coming from marginal portions of a visual field have angles as large as possible relative to an optical axis at a location of the pupil of the objective optical system. Since the conventional objective optical system has a large total length and the pupil which is located in the vicinity of the center thereof, a distance between a tip of the objective optical system and the pupil thereof is longer than the optical system for non-flexible endoscopes. When this optical system is used to compose a non-flexible endoscope for oblique observation, rays are high on a visual field direction changing prism which is disposed on a side of a tip of the objective optical system and eclipsed, whereby eclipse at a location apart from the pupil undesirably darkens and ununiformalizes brightness of an image.
The conventional single relay cycle type optical system for non-flexible endoscopes has the defect that it cannot have a wide field angle and is incompatible with non-flexible endoscopes for oblique observation as described above.